/*
You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
*/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        ListNode prev(-1), *cur = &head;
        if (!l1) return l2;
        if (!l2) return l1;
        int extra = 0;
        while(l1 || l2) {
            int sum = extra;
            if (l1) sum += l1->val;
            if (l2) sum += l2->val;
            extra = sum / 10;
            // store values inside new node
            cur->next = new ListNode(sum % 10);
            cur = cur->next;
            if (l1) l1 = l1->next;
            if (l2) l2 = l2->next;
        }
        if (extra) {
            ListNode *n = new ListNode(extra);
            cur->next = n;
        }
        return head;
    }
};
